The
rate of quail reproduction in the more arid portions of
their ranges is closely related to the amount of rainfall.
For example, in very dry years on the King Ranch of Texas,
most Northern Bobwhites do not breed and populations drop to
low levels. When rains are ample, however, the birds
reproduce normally. In semiarid parts of California, those
years with enough rain to produce spectacular displays of
wild flowers also tend to be good years for the reproduction
of California Quail.

But how do the rains
regulate the breeding of the quail? They seem to do so, in
part, by influencing the chemistry of the plants, especially
legumes (members of the pea family) that the quail eat. A
diet containing an extract from subterranean clover
("subclover" a low-growing clover that buries its seed heads
in the soil) has been shown to greatly reduce egg
productionby quail. That plant, among others, produces
"phytoestrogens," chemical compounds that are similar to the
hormones involved in regulating reproduction in birds and
mammals. The phytoestrogens may play a role in protecting
the plants against predation by herbivores. Their importance
in regulating the reproduction of animals that eat them was
first noted when subclover was found to inhibit the breeding
of Australian sheep.

Stunted plants of drought
years have higher phytoestrogen content than do plants with
the luxuriant growth of high-rainfall years. Much higher
levels of those compounds were found in samples of food
removed from the crops of California Quail in 1972; a
low-rain, low-reproduction year, than in samples from 1973,
a high-rain, high-reproduction year.

Other factors may also help
to regulate quail breeding. One is the quantity of
nutritious legume seeds available (they were almost three
times as abundant in the diet in 1973 as in 1972). Another
is rainfall itself, which could contribute some direct
stimulus for breeding. But diet appears to play the major
role, and that long-evolved role may, ironically, end up
depressing the size of some quail populations. Subclover, an
exotic species probably originating in the Mediterranean
region, is being used increasingly as a forage crop in
California. It is possible that this human-induced change
will reduce breeding in California Quail even in places and
years with more than adequate rainfall.